The two most common reasons for veganism/ vegetarianism seem to be
1) Moral reasons
I used to be vegetarian and now I'm vegan. I stopped eating meat and fish because I have always disliked most meats/fishes. As a result, my parents would physically force me to eat them (in some cases until I threw up). As a result I hate these foods even more.
I became vegan because I randomly became lactose intolerant a couple of years ago. I thought: "You don't eat meat, fish and now dairy is out of the question. Just drop eggs too, you rarely eat eggs anyway."